The kinetics of the oxidation of thiourea by 12-tungstocobaltate(iii) ion: evidence for anionic, neutral and protonated thiourea species in acetic acid–acetate buffer and perchloric acid solution

Abstract

The kinetics of the redox reaction between 12-tungstocobaltate(III) ion, [Co(III)W]⁵⁻, and thiourea is studied in acetic acid–acetate buffered solutions (3.4 ≤ pH ≤ 5.6), and dilute perchloric acid solution. The reaction in buffered solution is first order both in [Co(III)W]⁵⁻ and low [H₂NCSNH₂]. A simultaneous first- and second order dependence is observed at high [H₂NCSNH₂]. The rate is independent of pH ≤ 4.6 due to the participation of H₂NCSNH₂. In solutions of pH ≥ 4.6, the rate increases with the pH due to the dissociation of H₂NCSNH₂ to the reactive H₂NCSNH⁻ ion. In dilute perchloric acid solutions the rate increases with increasing [H⁺] due to the participation of protonated NH₂C⁺SHNH₂ species. The seat of the reaction is thought to be the enolic S atom (not protonated S) rather than the protonated nitrogen of the NH₂ group as in the oxidations of NH₂OH, H₂NNH₂ and N₃H where the rate is retarded by the increase in [H⁺]. The acid dissociation constant, K_a, of protonated NH₂C⁺SHNH₂ is estimated to be 0.048 mol dm⁻³ at 40 °C. The Marcus theory is used to estimate the self-exchange rate (k₂₂) of the H₂NCSNH₂–(H₂N)₂CSSC(NH₂)₂ couple. The estimated k₂₂ is 2.44 × 10⁻¹⁰ dm³ mol⁻¹ s⁻¹. The low value is attributed to the stable dimeric (H₂N)₂CSSC(NH₂)₂ species.